Which of the following is NOT classified as a penicillinase-resistant penicillin?

Amoxicillin is the correct choice as it is not classified as a penicillinase-resistant penicillin. This classification typically includes drugs that are designed to be resistant to the enzyme penicillinase, which is produced by certain bacteria to inactivate penicillin antibiotics. Nafcillin, oxacillin, and dicloxacillin are specifically formulated to withstand the activity of penicillinase, allowing them to effectively target penicillin-resistant strains of bacteria, particularly Staphylococcus aureus.

In contrast, amoxicillin is more broadly used in treating a variety of infections but is susceptible to degradation by penicillinase. Therefore, while amoxicillin is an effective antibiotic, it lacks the specific structural modifications that make the other options resistant to penicillinase, making it the choice that does not belong in the category of penicillinase-resistant penicillins.